1. Field of the Invention
This invention relates to a coating composition containing barium sulfate having an aminosilane surface treatment, and methods of using the coating composition as a protective coating.
2. Description of the Related Art
Protective surface coatings are used in a wide variety of applications to provide a thin film barrier between the surface of a body needing protection and its immediate surrounding environment. Protective coatings of this sort have been used, for instance, on marine, aircraft, and industrial structures and parts. These protective coatings often are formulated to include a curable organic medium, pigments, and inorganic filler particles dispersed within the medium.
It is well known that the proper selection of the pigments, as differentiated from the fillers, has a profound influence on protective and other functional properties of protective coatings. The pigments provide many of the essential properties of the coating such as color, corrosion protection, durability and special rheological properties that address the practical aspects of coating application such as ease of application and firm build. Many conventional fillers used in coatings are commodities having lower cost than the base resin of the coating. For this reason, fillers are often used to reduce the cost of the coating.
The traditional coating requirements of increased performance, reduced cost, as well as compliance with regulations drive much of new coatings formulation, and are largely responsible for the elimination of the older thermoplastics (lacquers) and the increase in higher solids thermosets and water borne technologies, as well as more revolutionary advances.
In particular, regulation compliance is driving two of the most important coating formulations changes, which are (1) the elimination of corrosion resistant inhibitive coating systems based on lead and hexavalent chromium because of toxicity considerations, and (2) the reduction in the volume used of volatile organic compound (VOC) solvents and diluents, which are released directly into the atmosphere in many coating applications. Any coating additive that attenuates the need for toxic corrosion inhibitors or permits lower solvent and diluent usage, and thus permits use of lower levels of VOCs, is highly desired.
The more recent coatings have serious drawbacks. For example, the more recently introduced safer corrosion inhibitors are either not as effective or as universally applicable as the traditional corrosion inhibitors, such as lead and the hexavalent salts of chromium. This has resulted in a swing away from coating systems based on inhibitive pigments toward coatings that incorporate sacrificial pigments such as zinc. This second approach also has limitations. Zinc-rich technologies require good contact between the steel of the substrate and this limits these systems to new steel or old steel that has been blasted clean. Old steel that is covered with lead and chromium based coatings must first be blasted clean which undesirably puts lead and chromium debris into the environment. Attempts to contain the debris and its removal and disposal as hazardous material is excessively costly and severely impedes any impetus towards such surface preparation and the use of such zinc-rich coatings on steel covered with lead or chromium based coatings.
A third technique to combat corrosion is the barrier technique. Barrier coatings protect metallic substrates by interposing an oxygen and ionic barrier between the substrate and the environment and ensures that any water that does penetrate the film is filtered of all ionic material so that the electrical resistance of any underfilm electrolyte is too high to allow the establishment of a corrosion current.
Barrier coatings have traditionally been formulated with flat platy pigments (aluminum and stainless steel flake, mica, micaceous iron oxide, talc, glass flake etc.). The flat, platy pigment shape is believed to enhance the barrier properties of the coating. Unfortunately, many of these pigments have two important defects. First, they are often reactive and sensitive to various chemical species. For example, aluminum is sensitive to acids and alkalis, while glass flake may be affected by alkalis. Secondly, they are notoriously high in oil absorption because of their high surface area and therefore make high viscosity coatings that cannot be applied without large solvent additions (high VOC).
Therefore, a need still exists for enhanced barrier and corrosion performance with respect to resin-based thin film forming protective coatings while also providing acceptably low VOC contents and reduced health and environmental risks.
The use of barium sulfates pretreated with organosilanes in bulk thermoplastic polymers has been described, such as in U.S. Pat. No. 6,194,070 and by Wang, K., et al., Effect of Interfacial Interaction on Theological and Crystalline Behavior of Polypropylene/BaSO4 Composites, ACTA Polymerica Sinica, No. 6, Dec. 2001, 697-700.